1. Field of the Invention
This invention generally relates to the field of coating apparatus for conductive pastes, and more particularly, to a method and apparatus for extruding a conductive paste through a stencil or screen.
2. Discussion of Related Art
In the fabrication of substrates and carriers for microelectronic components such as integrated circuit chips, the screen printing of lines of conductive paste onto substrates or sheets is a well known technique. In practice, a mask is placed in intimate contact with a substrate, and a conductive paste is then applied. Screening masks are typically thin sheets of metal such as molybdenum, steel, or copper, having a pattern of voids or openings through which the paste is passed. The voids correspond to the desired pattern of lines to be formed on a device.
There are several well known, general, techniques for applying conductive paste. For example, conductive paste may be deposited in a puddle or mass on screening mask, and a squeegee blade moved across the mask to force the paste into the stencil openings and on to the substrate. Alternatively, the screening mask is contacted with a nozzle having an elongate opening for extruding paste into the stencil openings. The nozzle may comprise an integral squeegee blade for wiping, or a separate blade may be used.
Nozzles and methods for their use are also known. The following references illustrate the state of the pertinent art.
U.S. Pat. No. 4,622,239 to Schoenthaler, et al., discloses a nozzle apparatus having elastomeric blades.
U.S. Pat. Nos. 4,636,406 and 4,693,209 to Leicht disclose a method and nozzle apparatus for dispensing solder paste.
U.S. Pat. No. 4,665,723 to Zimmer discloses a nozzle apparatus for dispensing a liquid on a moving web.
U.S. Pat. No. 5,387,044 to Accardo discloses a nozzle apparatus comprising a removable squeegee blade.
U.S. Pat. No. 5,478,700 to Gaynes, et al., discloses a nozzle apparatus having a pair of flexible nozzle lips.
Finally, S. W. Cornell, et al., disclose in the IBM Technical Disclosure Bulletin, Vol. 14(3), 739, (August 1971) a nozzle apparatus having a Teflon nozzle insert.
Much of the prior art has certain deficiencies or disadvantages. One disadvantage is damage and wear to masks caused when hard materials are used to fabricate the contacting element of some nozzles. Another disadvantage is the high-cost associated with fabricating nozzle contacting elements to very tight tolerances to maintain an effective extrusion seal. A third disadvantage is the loss of paste through nozzle end leakage during screening. Additionally, many known nozzle designs necessitate continuous rapid cleaning and drying of masks after screening in a separate step. Finally, the complete filling of deep or high aspect ratio holes or vias is often difficult. Extrusion screening processes are affected by many variables. The changing of certain process variables to achieve a complete via fill is not always successful or predictable. For example, it is possible to increase wipe time, however screening tool throughput is lowered. Paste pressure may be increased, however the seal between the nozzle and the screen or mask is not maintained and paste leaks result. Lowering paste viscosity may result in poor quality screened lines. Thus, the adjusting of the aforesaid parameters is not desirable.
It would thus be desirable to provide an improved nozzle apparatus and method for use which provides a solution to the aforesaid and other deficiencies and disadvantages.